Paul Williams, an inescapable presence in the 1970s and the songwriter behind the Muppets’ “Rainbow Connection,” is the subject of this insightful documentary.Abramorama

A ubiquitous TV game and talk-show presence who racked up 50 appearances on Johnny Carson’s “Tonight Show” and a lengthy list of acting credits, including in the “Smokey and the Bandit” trilogy, Paul Williams was virtually inescapable in the 1970s and early 1980s.

The chubby, diminutive singer also wrote a string of era-defining hits, among them “We’ve Only Just Begun” (for the Carpenters), “An Old-Fashioned Love Song” (Three Dog Night) and “Rainbow Connection” (the Muppets), and shared an Oscar with Barbra Streisand for “Evergreen.’’

But Williams disappeared so thoroughly that Stephen Kessler (“Vegas Vacation”), the director of this unusual and unexpectedly inspiring documentary, was shocked to learn that Williams — whose career was derailed at its height by drugs and booze — is still working, albeit in a far less high-profile fashion.

“Paul Williams: Still Alive” is culled from a treasure trove of often cringe-worthy archival clips (like his skydiving on “Circus of the Stars”), and two years’ worth of new footage (including a singing tour in the Philippines) Kessler shot of Williams, two decades sober and no longer hiding enlarged pupils and bloodshot eyes behind his trademark tinted glasses.

Williams, who was elected president of ASCAP in 2009, speaks frankly and eloquently about his problems dealing with fame, and his recovery. And more important, he earns our thanks by resolutely refusing to let Kessler turn this into a clichéd documentary.